The present invention relates generally to a new and improved design of a plant for the treatment and removal of contaminated gases formed during the production of granulated slag. More particularly, this invention relates to a design of a plant for the treatment of a mixture of steam and air contaminated with sulphurous gases formed during the granulation process of blast furnace slag.
A number of processes and plants are known at the present for the granulation of blast furnace slag. One such known process, for example, is to inject a powerful jet of water into a flow of molten slag with the aid of a spraying head. Advantageous embodiments of such spraying heads are described, for example, in European Patent Application EP 0,082,279.
A control problem in a plant of this type is the steam contaminated, among other things, with sulphurous gases including hydrogen sulphide, H.sub.2 S and sulphur dioxide, SO.sub.2, which are generated in large quantities and at essentially variable rates when the water is injected into molten slag.
After the granulation the slag can be dehydrated, for example in a rotary cylinder bounded externally by a filtering surface. This process and the corresponding plant are described in U.S. Pat. No. 4,204,855 which is incorporated herein by reference. During this dehydration of slag, a mixture of steam and air, which is contaminated with sulphurous gases, is also formed.
The contaminated vapors produced during the granulation are sometimes discharged untreated into the atmosphere, which creates a health hazard and constitutes a nuisance for the inhabitants dwelling near the factories, particularly in view of the unpleasant odor and toxicity of H.sub.2 S (hydrogen sulphide).
Some plants are equipped with a closed condensation tower which is situated above the granulation basin in which the slag is cooled. In this condensation tower the hot gases and vapors rising by convection are sprayed with water by jets. The water spray originating from the jets and the condensates are recovered by chutes situated below the jets. However, these plants are not entirely satisfactory. Since the flow rates of slag and consequently the flow rates of ascending vapors and gas can vary as a function of time, it is difficult to create and maintain a stable and sufficient pressure reduction by condensing the steam in the region of the jets. At certain times an overpressure is thus created in the condensation region of the tower, which prevents the vapors from rising as far as the jets. Under these conditions, sufficient condensation of the steam and, thus, effective removal of the sulphurous gases is no longer ensured. Contaminated vapors can then escape uncontrollably into the open air.
German Patent Application P 3511958.6, filed on Apr. 2, 1985 describes a plant for the granulation of blast furnace slag which employs a closed gas circulation system. The vapors generated when water is injected into the molten slag are entrained by a jet of water directly into the water-filled granulation basin, to be partially condensed therein. The residual vapors and gases given off from the granulation basin are sprayed with water with the aid of jets installed in a condenser situated above the water reserve. The rising vapors and the sprayed water consequently form two countercurrent streams. It is claimed that the residual gases and vapors which are not removed by this scrubbing are led back towards the spray head by an internal circulation, which is said to be created in the plant. The German document does not mention how to avoid an overpressure in the plant, which gives rise to an uncontrolled leakage of contaminated gases to the outside. Nor can this system be employed for retro fitting to an existing plant. The granulation plant must, in fact, be constructed so as to form a perfectly gastight system.
There is an obvious need for an improved process and plant for the treatment of a mixture of air and noxious vapors formed during the production of granulation of blast furnace slag and contaminated with sulphurous gases. In addition, a process is needed to treat effectively widely variable flow rates of contaminated vapors and which prevents untreated air or vapors from escaping to the outside of the plant.